The present invention relates to a polysulfone semipermeable membrane that has a specific membrane structure and a method of manufacturing the same by a wet phase conversion method using a specific solvent. More particularly, the invention relates to a membrane for gas separation, especially a polysulfone semipermeable membrane used for separating and concentrating specific components such as hydrogen, methane, carbonic acid gas, oxygen, nitrogen, steam, and acid gas from industrial mixed gases, or used for water processing, etc., and a method of manufacturing the same.
Polysulfone and a derivative thereof, etc. have been known as separation membrane materials that are inexpensive, chemically stable, and excellent in various properties such as mechanical strength and resistance. Such materials also have been examined in various fields of the separation membranes. Examples of the materials are disclosed in Laid-open Japanese Patent Application No. (Tokkai hei) 3-196824, Laid-open Japanese Patent Application No. (Tokkai hei) 3-72927, Laid-open Japanese Patent Application No. (Tokkai hei) 3-154625, Laid-open Japanese Patent Application No. (Tokkai hei) 1-81823, Laid-open Japanese Patent Application No. (Tokkai hei) 1-58324, U.S. Pat. No. 5,049,169, and U.S. Pat. No. 5,071,448.
Furthermore, considering a separation membrane having practical mechanical strength, a method of manufacturing a thin film or an asymmetric membrane has been examined. Examples of methods of manufacturing a thin film of polysulfone resin include methods of coating to a porous support membrane disclosed in U.S. Pat. No. 5,009,678, U.S. Pat. No. 4,941,893, etc., and methods of manufacturing an asymmetric membrane disclosed in U.S. Pat. No. 4,880,441, J. Appl. Polym. Sci., 40, 1557 (1990), J. Appl. Polym. Sci., 40, 1575 (1990), J. Appl. Polym. Sci., 41, 713 (1990), etc. However, when polymers having a high performance separation factor are formed on a suitable porous support membrane as a thin film, the thickness of the thin film should be desirably 0.1 .mu.m or less so as to increase the permeation speed of gas to a practical level. Manufacturing such a thin film industrially results in complicated manufacturing processes, low yield, and high cost, and therefore it is not suitable for industrial practice. While sputtering etching (Laid-open Japanese Patent Application No. (Tokkai-sho) 57-140608) and a method of manufacturing an asymmetric membrane are also described in the above literature, it was difficult to industrially manufacture a membrane having a required thickness of 0.1 .mu.m or less without a pinhole. U.S. Pat. No. 4,929,405 discloses a method in which the thickness of a homogeneous membrane of aromatic polyimide containing fluorine is controlled to 400 angstrom (40 nm) or less, which is a required thickness of 0.1 .mu.m or less, by a method of spreading on a water surface (Langmuir-Blodgett). However, manufacturing such a membrane industrially was difficult.
In the above conventional methods, when manufacturing a semipermeable membrane practically and industrially using an inexpensive polysulfone resin to perform efficient separation operation, the above problems arose. Therefore it was difficult to obtain a satisfactory membrane structure, especially a gas separation membrane structure. Particularly, for a composite membrane, a thin film is formed on a porous support membrane, so that the dynamic strength at the interface is not enough. Therefore, in order to form a thin film without a defect which has gas separation function, filling in the pores of the porous support membrane, the thickness must be thick. For an asymmetric membrane, the membrane is integrally formed of a skin layer and a porous layer, so that no separation occurs at the interface, and the dynamic strength is enough. However, it was difficult to industrially form a skin layer without a defect which has gas separation function and has a thickness of 100 nm or less.
In order to solve the conventional problems as mentioned above, the present invention provides a polysulfone semipermeable membrane in which a gas separation membrane that has high permeation flux and can be produced for a satisfactory cost is formed by a more simple method. The present invention also provides a method of manufacturing the same.